Let suppose A, B and C are the angles of a triangle ABC,

In ∆ABC,

∠ A = (a – d)

∠ B = a

∠ C = a + d

Now, form an increasing A.P

As we know Sum of all the angle of a triangle is 180°,

Therefore,

∠A + ∠B + ∠C = 180°

(a – d) + a + (a + d) = 180°

3a = 180°

a = 180/3 = 60°

From the table,

sin B = sin A = sin 60°

= √3/2